1. Field of the Invention
This invention relates to integrated circuits and integrated circuit structures, and in particular, to a technique for suppressing electromigration in conducting lines formed on integrated circuit structures.
2. Description of the Prior Art
The phenomenon of electromigration, that is, transfer of material along a conducting line, in integrated circuit technology is well known. Although the phenomenon is not completely understood, it is widely believed that material is moved along the electric conductor by the presence of the electric field and electron flow momentum. One measure of the ease of electromigration in a given material formed under given conditions is the electromigration activation energy, which is a quantitative measure of the potential energy barrier heights that the electromigrating atoms must overcome to move from one position in a lattice to another. The electromigration activation energy is related to the activation energies for ordinary thermal diffusion. In an aluminum film the activation energy usually observed for electromigration is 0.5 to 0.7 electron volts (herein eV). The activation energy for bulk diffusion in aluminum is approximately 1.3-1.5 eV.
It is known that electromigration in polycrystalline films proceeds substantially along grain boundaries. Grain boundaries are disordered regions between the crystalline grains which offer an easier path for diffusion of the atoms being transported. The grain boundaries are characterized by a lower activation energy for diffusion than that for the bulk or crystalline grain.
Some electromigration experiments have been published in which single crystal aluminum films were prepared by epitaxial techniques to create an entire conducting line comprised of a single crystalline grain. Films fabricated in such a manner exhibit drastically reduced electromigration compared with ordinary polycrystalline films, and in fact have activation energies characteristic of bulk diffusion, for example, 1.3 eV. See, for example, F. d'Heurle and I. Ames, "Electromigration in Single Crystal Aluminum Films," Appl. Phys. Lett. 16, 80 (1970). Single crystal films, however, are obviously impractical to produce on actual integrated circuits in production quantities.